The present invention relates to error detection and correction, and more particularly to such correction as applied to burst errors.
Recently the field of digital television technology has been developed. In this technique, a video signal is sampled, typically at four times the color subcarrier frequency. Each sample ("pixel") is digitized, usually using an 8-bit code word, which provides 256 grey levels. Frequently, it is then desired to magnetically record the code words.
In the magnetic tape recording of digital data, various channel codes are used to perform the actual recording of the data. These channel codes transform one set of source waveforms, e.g. the above-described 8-bit code words, into another uniquely decodable set of waveforms which are better suited to the nonlinearities encountered in the magnetic tape channel. These channel codes are well known, and generally fall into two categories, phase codes and group codes. Phase codes alter the phase of transitions in the recorded signal as a function of the data to be recorded. Group codes are a one-to-one mapping between a set of source bits and a corresponding tape waveform.
A major consideration in recording is to minimize the DC content of the recorded waveform, because of the tape recorder's poor ability to recover low frequency information. In view of this, a new group code was recently designed by the Independent Broadcast Authority (IBA) specifically for the problem of recording digital video signals. The IBA found that there are 252 different combinations of 10 bits which have an equal number of 1's and 0's, that is, have a DC content of exactly zero. Thus, the IBA code is one which maps a data word consisting of 8 "source bits" to an on tape waveform with 10 "tape bits" per word, that is, 10 transitions. This is accomplished in such a manner so that the 256 on-tape words correspond to the 252 "no-DC" 10-bit words and 4 additional 10-bit words.
A simplified example is given below. In this example, it is assumed that 3-bit words are used for the digitization of the video samples (8 gray levels) and 6 bit "no DC content" words are used for the recording code.
______________________________________ DIGITIZATION WORDS RECORDING WORDS ______________________________________ 000 000111 001 111000 010 101010 011 010101 100 001011 101 110100 110 001110 111 110001 ______________________________________
It should be noted that all of the recording words have an equal number of ones and zeros, i.e. have no DC content. The next higher even order code word length from the 3-bit source code word is, of course, 4-bits. However, there are only 6 no-DC code words among all of the 16 4-bit code words. Therefore it is necessary to go to the next higher order even order code word length, which is 6-bits. There are twenty no-DC code words among the 64 possible words in the group of 6-bit words, which is more than enough for the eight possible 3-bit source code words.
In addition to the poor low frequency response described above, another problem with magnetic recording and playback is errors, which errors usually occur in bursts as a result of dropouts. Since in the above-identified 6-bit recording word code, only eight out of a possible 64 6-bit code words are valid code words, seven eighths (7/8) of all possible errors can be detected, this still leaves the remaining one-eighth (1/8) of the errors undetected. Similarly, in the 10-bit IBA code about 2.sup.8 /2.sup.10 =3/4 of all errors are detected leaving the 1/4 of the errors undetected.
It is therefore desirable to conceal substantially all errors in a data channel and not just the detected errors.